1. Field of the Invention
The present invention relates to an optical recording medium having a superior durability, in particular, high-temperature durability, and a process for producing the optical recording medium. The present invention also relates to an information recording method.
2. Related Background Art
In recent years, optical recording mediums, recording and reproducing information using light, are proposed as a means for storing a vast amount of information. Conventionally known optical recording methods include, as disclosed in "Review and Analysis of Optical Recording Media" in Optical Engineering, Vol.15, No.2, March-April 1976, pp.99ff., those types in which a recording layer is irradiated with a light beam, e.g., laser light, to cause deformation, hole or bubble production or changes in optical properties in the recording layer.
Materials used for the recording layer are known to include, for example, low-melting metals such as Te, Bi, Sn, Sb and In, dye and pigments (organic coloring matters) such as cyanines, squariums, phthalocyanines, tetradehydrocholines, polymethines, naphthoquinones and benzenedithiol nickel complexes, and composite materials comprised of any of these organic coloring matters and metals.
Of the above recording layer materials, organic coloring matters have been attracting notice in recent years as inexpensive optical recording mediums. More specifically, in organic coloring matter optical recording mediums, the recording layer can be formed on a substrate by solvent coating, and hence such optical recording mediums can promise a better mass-productivity than low-melting metal compound optical recording mediums whose recording layers are formed by vacuum deposition.
In the optical recording medium utilizing the organic coloring matter in the recording layer, the recorded information can be detected because the reflectance at the recording pits decreases as a result of decoloring caused by, for example, thermal decomposition of the organic coloring matter as well as the change of the reflected light from the information reproducing laser light at the recording pits caused by the effect of optical scattering due to the deformation of the recording pits.
In recent years, thinnes has been required for optical recording mediums as the recording-reproducing apparatus becomes small-sized. Optical cards, having the shape of cards, are required to have a thickness of from 0.68 to 0.84 mm according to the ISO standard.
On the other hand, not to be effected by dust or scratches on the substrate surface of the optical recording medium, it is more advantageous for the substrate to be as thick as possible. Usually thickness of about 0.4 to 0.6 mm is suitable. Protective substrates are required to have a thickness of about 0.15 to 0.4 mm. Hence, it is difficult to provide in the optical card having the above thickness what is called the air-gap structure, a structure in which a void space with a gap of about 0.5 to 1 mm is provided between a recording layer formed on a substrate and a protective substrate.
Under these circumstances, thin-type optical recording mediums such as optical cards have an adhered structure in which a protective substrate is laminated to a recording layer interposing an adhesive layer between them.
However, when an adhesive layer is formed directly on a recording layer containing an organic coloring matter (hereinafter "organic coloring matter recording layer"), sometimes the organic coloring matter reacts with an adhesive to cause deterioration of the recording layer, resulting in a decrease in the C/N of the information recorded in the recording layer.
To solve these problems, a laminate optical card is proposed in Japanese Laid-Open Patent Application No. 2-3116 which comprises a recording layer on which an optical information pattern is recorded, an adhesive layer and a spacer layer provided between the recording layer and the adhesive layer. In Japanese Laid-Open Patent Application No. 1-146144(U.S. Ser. No. 598,748, now U.S. Pat. No. 5,085,925), the present applicants have disclosed an optical recording medium making use of an adhesive mainly composed of an ethylene-acrylic acid copolymer, an ethylene-acrylate copolymer or an ethylene-maleic anhydride-acrylic acid terpolymer as an adhesive that does not damage the recording layer.
In the above techniques, however, the method providing the spacer layer has the disadvantage that it requires a complicated manufacturing process which brings about a high cost.
While the adhesive mainly composed of an ethylene-acrylic acid copolymer, an ethylene-acrylate copolymer or an ethylene-maleic anhydride-acrylic acid terpolymer has less interaction with the organic coloring matter in the recording layer, and is an excellent laminating adhesive for optical recording mediums. For the optical recording medium making use of this adhesive, there are still many requirements, for example, the reduction of reflectance of the organic coloring matter recording layer should be minimized to the greatest extent when the optical recording medium is stored in an environment of high temperature and high humidity, e.g., in an environment of 70.degree. C. and 90% RH for 100 hours, or when a high power beam is used as the reproducing light beam. Stated specifically, change rate of reflectance in an environment of 70.degree. C. and 90% RH should be controlled to be 10% or less, particularly 6% or less, and more particularly 4% or less. Also, when continuously irradiated with the high power reproducing light beam, it is desirable that 10% reduction of the reflectance of a recording layer does not occur by 200 seconds or more, particularly 250 seconds or more, and more particularly 280 seconds or more.